The present invention relates to composites and composite materials, more particularly to methods and configurations for joining composite structures to other composite structures or to non-composite structures.
Many structural applications require the joining of composites to composites, or composites to metals. These kinds of joints are often technologically challenging. Conventional approaches to joining composites with other composites or with metals include mechanical fastening (e.g., bolting), adhesive bonding, co-curing, and secondary bonding. A common design for mechanical fastening is a lap joint, which is typically formed by overlapping two panels, then match-drilling holes in the two panels, and then inserting bolts to fasten together the two panels. A common design for adhesive bonding is a scarf joint, which is typically formed by matching the respective tapered edges of two panels, and applying an adhesive material so as to achieve a uniform thickness joint having an adhesive bond line between the two matched tapered edges. Scarf joints also lend themselves to co-curing or secondary bonding, either of which obviates the need for adhesive bonding. According to typical co-curing technique, a joint involving two uncured panels is laid up and cured in one step. According to typical secondary bonding technique, an uncured panel is laid onto a previously cured panel and attached thereto via a second cure.
The aerospace industry has considerable experience with fabrication of composite-to-metal joints. In aerospace structure technology, mechanical fastening and adhesive bonding are the two most popular approaches to joining composite materials with metal materials. Prevalent in aerospace applications is a “bolted-bonded” configuration, in which mechanical fastening and adhesive bonding are combined to create redundant load paths in a structure. Bolted-bonded configurations are also seen in marine applications involving the joining of composite components to metallic structures; however, adhesives are susceptible to degradation in aqueous environments. An adhesive bond entails not one but two interfaces that are prone to disbonding, namely, the respective interfaces between the adhesive layer and the two adherends. Therefore, regardless of whether it is used alone or in combination with mechanical fastening, adhesive bonding is viewed much less favorably in the marine realm than it is in the aerospace realm. Moreover, for many marine structures, the sizes and shapes of the structural sub-assemblies prohibit the use of either co-curing or secondary bonding as a joining technique. Accordingly, mechanical fastening (e.g., bolted joints) has been widespread in the marine industry as an exclusive joining technique.
Mechanical fastening can be utilized to great benefit but has several drawbacks. Since composites tend to be sensitive to damage under high bearing pressures, a lap joint must be carefully designed in order to carry the intended loads without accumulating damage in the vicinity of the bolt-holes in the composite. Commentators have cautioned that maintaining close fit-up between the holes and the bolts, and between the members being joined, is important for maximizing fatigue performance. Some composites also exhibit low temperature creep that leads to loss of preload in the bolts and accelerates damage, a proclivity that may necessitate regular maintenance of the bolted joints to maintain preload. The advantageousness of composites in terms of weight savings may be vitiated by bolted joints because of the weight of the bolts, and because the composite panel thickness is often increased in the vicinities of the bolted joints to decrease the bearing stresses in the composite.
Bolted lap joints may be impractical for outer hull applications where hydrodynamics (or aerodynamics) is a consideration, because a simple lap joint entails at least one “step” (where the lapped panels overlap) on the hull structure's surface. Although bolted flange joints can be used for attachment of hull sections, these are significantly heavier than bolted lap joints. A tapered lap joint configuration (in which the panels of a lap joint are tapered on the edges) can be adopted so as to ameliorate the negative effects of the overlaps on the hull structure's hydrodynamic (or aerodynamic) characteristics. As distinguished from a tapered lap joint, the above-mentioned scarf joint matches (interfaces) the tapered edges of panels so as to achieve a uniform thickness joint; typically, adhesive bonding is implemented where the respective tapered edge surfaces of the panels are matched up. A scarf joint, if properly designed, can achieve a uniform shear stress in the bond line, thus representing a highly efficient joint. Theoretically, at least, the potential efficiency of an adhesive joint is superior to that of a mechanical joint, since an adhesive joint is theoretically capable of achieving one hundred percent of the laminate strength. Nevertheless, as previously noted herein, marine use of adhesive bonding can be problematical due to the tendency of adhesive materials to degrade in aqueous environments.